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{{About|the dwarf planet}} Not a planet.
<!--
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{{Infobox Planet
| bgcolour=#A0FFA0
| name = Pluto
| symbol = ]
| image = ]
| caption = Computer-generated map of Pluto from ] images, synthesised true colour<ref group=note>The HST observations were made in two wavelengths, which is insufficient to directly make a true colour image. However, the surface maps at each wavelength do limit the shape of the ] that could be produced by the materials that are potentially on Pluto's surface. These spectra, generated for each resolved point on the surface, are then converted to the ] colour values seen here. See Buie et al, 2010.</ref> and among the highest resolutions possible with current technology
| discovery = yes
| discoverer = ]
| discovered = February 18, 1930
| mp_name = '''134340 Pluto'''
| named_after = ]
| mp_category = ],<br />],<br />],<br />],<br />]
| epoch = ]
| aphelion = 7,375,927,931 km<br />49.305&nbsp;032&nbsp;87 ]
| perihelion = 4,436,824,613&nbsp;km<br />29.658&nbsp;340&nbsp;67 AU
| semimajor = 5,906,376,272&nbsp;km<br />39.481&nbsp;686&nbsp;77 AU
| eccentricity = 0.248&nbsp;807&nbsp;66
| inclination = 17.141&nbsp;75°<br />11.88° to Sun's equator
| asc_node = 110.303&nbsp;47°
| arg_peri = 113.763&nbsp;29°
| period = 90,613.305 days<br />248.09 ]<br />14,164.4 Pluto ]s<ref name="planet_years">{{cite web|url = http://cseligman.com/text/sky/rotationvsday.htm|title = Rotation Period and Day Length|last = Seligman |first = Courtney|accessdate = 2009-08-13}}</ref>
| synodic_period = 366.73 days
| avg_speed = 4.666&nbsp;km/s
| satellites = ]
| physical_characteristics = yes
| mean_radius = {{nowrap|1,153 ± 10 km}}<ref name="Buie06"/><br />0.18 Earths
| surface_area = 1.665{{e|7}} km<sup>2</sup><ref group=note>Surface area derived from the radius ''r'': <math>4\pi r^2</math>'''.</ref><br />0.033 Earths
| volume = 6,39{{e|9}} km<sup>3</sup><ref group=note>Volume ''v'' derived from the radius ''r'': <math>4\pi r^3/3</math>'''.</ref><br />0.0059&nbsp; Earths
| mass = (1.305 ± 0.007){{e|22}} kg<ref name="Buie06"/><br />0.002&nbsp;1 Earths<br />0.178 moon
| density = 2.03 ± 0.06 g/cm<sup>3</sup><ref name="Buie06"/>
| surface_grav = {{Gr|13.05|1153}} ]<ref group=note>Surface gravity derived from the mass ''m'', the ] ''G'' and the radius ''r'': <math>Gm/r^2</math>.</ref><br />0.067 ]
| escape_velocity = {{V2|13.05|1153}}&nbsp;km/s<ref group=note>Escape velocity derived from the mass ''m'', the ] ''G'' and the radius ''r'': {{math|{{radical|2Gm/r}}}}.</ref>
| sidereal_day = ]<br />6 d 9 h 17 m 36 s
| rot_velocity = 47.18&nbsp;km/h
| axial_tilt = 119.591 ± 0.014° (to orbit)<ref name="Buie06">{{cite journal
| author = M. W. Buie, W. M. Grundy, E. F. Young, L. A. Young, S. A. Stern
| title = Orbits and photometry of Pluto's satellites: Charon, S/2005 P1, and S/2005 P2
| journal = Astronomical Journal
| year=2006
| volume=132
| pages=290
| url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2006AJ....132..290B&amp;db_key=AST&amp;data_type=HTML&amp;format=&amp;high=444b66a47d27727
|id = {{arxiv|archive=astro-ph|id=0512491}} | doi = 10.1086/504422
}}</ref><ref name=inclination group=note>Based on the orientation of Charon's orbit, which is assumed the same as Pluto's spin axis due to the mutual ].</ref>
| right_asc_north_pole = 133.046 ± 0.014°<ref name="Buie06"/>
| declination = −6.145 ± 0.014°<ref name="Buie06"/>
| albedo = 0.49–0.66 (varies by 35%)<ref name=Hamilton>{{cite web
|date=2006-02-12
|title=Dwarf Planet Pluto
|publisher=Views of the Solar System
|author=Calvin J. Hamilton
|url=http://www.solarviews.com/eng/pluto.htm
|accessdate=2007-01-10}}</ref><ref name="Pluto Fact Sheet"/>
| magnitude = 13.65<ref name="Pluto Fact Sheet"/> to 16.3<ref name=AstDys-Pluto>{{cite web
|title=AstDys (134340) Pluto Ephemerides
|publisher=Department of Mathematics, University of Pisa, Italy
|url=http://hamilton.dm.unipi.it/astdys/index.php?pc=1.1.3.1&n=134340&oc=500&y0=1870&m0=2&d0=9&h0=0&mi0=0&y1=1870&m1=3&d1=20&h1=0&mi1=0&ti=1.0&tiu=days
|accessdate=2010-06-27}}</ref><br/>(mean is 15.1)<ref name="Pluto Fact Sheet"/>
| abs_magnitude = −0.7<ref name=jpldata>{{cite web
|title=JPL Small-Body Database Browser: 134340 Pluto
|url=http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=Pluto
|accessdate=2008-06-12}}</ref>
| angular_size = 0.065" to 0.115"<ref name="Pluto Fact Sheet"/><ref group=note>Based on geometry of minimum and maximum distance from Earth and Pluto radius in the factsheet</ref>
| pronounce = {{IPAc-en|en-us-Pluto.ogg|ˈ|p|l|uː|t|oʊ}},<ref group=note>In US dictionary transcription, {{USdict|plōō′·tō}}. From the {{lang-la|Plūto}}</ref>
| adjectives = Plutonian
| atmosphere = yes
| temperatures = yes
| temp_name1 = ]
| min_temp_1 = 33 K
| mean_temp_1 = 44 K
| max_temp_1 = 55 K
| surface_pressure = 0.30 ] (summer maximum)
| atmosphere_composition = ], ]
| note=no
}}
'''Pluto''', ] '''134340 Pluto''', is the second-largest known ] in the ] (after ]) and the tenth-largest body observed directly orbiting the ]. Originally classified as a planet, Pluto is now considered the largest member of a distinct population known as the ].<ref name=wiki-kbo group=note >Although Eris is larger than Pluto, it resides in the ]. Misplaced Pages convention treats this as a distinct region from the Kuiper belt, so Pluto becomes the largest Kuiper belt object.</ref>


Yep, definately not a planet
Like other members of the Kuiper belt, Pluto is composed primarily of rock and ice and is relatively small: approximately a fifth the mass of the ]'s ] and a third its volume. It has an ] and highly inclined orbit that takes it from 30 to 49&nbsp;] (4.4–7.4 billion&nbsp;km) from the Sun. This causes Pluto to periodically come closer to the Sun than ].

From its discovery in 1930 until 2006, Pluto was considered the Solar System's ]. In the late 1970s, following the discovery of minor planet ] in the outer Solar System and the recognition of Pluto's very low mass, its status as a major planet began to be questioned.<ref name=ridpath>{{cite journal|author=Ian Ridpath|title=Pluto—Planet or Imposter?|url=http://myweb.tiscali.co.uk/ianridpath/Pluto.pdf|journal=Astronomy|date=December 1978 |pages=6–11}}</ref>
In the late 20th and early 21st century, many objects similar to Pluto were discovered in the outer Solar System, notably the ] ] in 2005, which is 27% more massive than Pluto.<ref>{{cite web|title=Astronomers Measure Mass of Largest Dwarf Planet|work=hubblesite|year=2007|url=http://hubblesite.org/newscenter/archive/releases/2007/24/full/|accessdate=2007-11-03}}</ref> On August 24, 2006, the ] (IAU) ]. This definition excluded Pluto as a planet and added it as a member of the new category "dwarf planet" along with Eris and ].<ref>{{cite web
| url = http://news.bbc.co.uk/1/hi/magazine/4737647.stm
| title = Farewell Pluto?
| author = A. Akwagyiram
| publisher = BBC News
| date = 2005-08-02
| accessdate = 2006-03-05
}}</ref> After the reclassification, Pluto was added to the list of ]s and given the ] 134340.<ref>{{cite web
| url=http://cfa-www.harvard.edu/mpec/K06/K06R19.html
| title = MPEC 2006-R19 : Editorial Notice
| author = T. B. Spahr
| publisher = Minor Planet Center
| date = 2006-09-07
| accessdate = 2006-09-07}}</ref><ref>{{cite web
| url = http://www.newscientistspace.com/article/dn10028-pluto-added-to-official-minor-planet-list.html
| title = Pluto added to official "minor planet" list
| author = D. Shiga | publisher=]
| date = 2006-09-07
| accessdate = 2006-09-08}}</ref> A number of scientists continue to hold that Pluto should be classified as a planet.<ref>{{cite web
| url = http://www.telegraph.co.uk/earth/main.jhtml?xml=/earth/2008/08/10/scipluto110.xml
| title = Pluto should get back planet status, say astronomers
| author = Richard Gray
| publisher = The Telegraph
| date = 2008-08-10
| accessdate = 2008-08-09
}}</ref>

Pluto and its largest moon, ], are sometimes treated together as a ] because the ] of their orbits does not lie within either body.<ref>
{{cite journal
|title = The mass ratio of Charon to Pluto from Hubble Space Telescope astrometry with the fine guidance sensors
| journal = Icarus
| volume = 164
| pages = 254–259
| author = C.B. Olkin, L.H. Wasserman, O.G. Franz
| work = Lowell Observatory
| url = http://www.as.utexas.edu/~fritz/astrometry/Papers_in_pdf/%7BOlk03%7DPlutoCharon.pdf
| year = 2003
| doi = 10.1016/S0019-1035(03)00136-2
| accessdate = 2007-03-13
}}</ref> The IAU has yet to formalise a definition for binary dwarf planets, and until it passes such a ruling, they classify Charon as a ] of Pluto.<ref>
{{cite web
| title = The Path to Defining Planets
| author = O. Gingerich
| work = Harvard-Smithsonian Center for Astrophysics and IAU EC Planet Definition Committee chair
| year = 2006
| url = http://astro.cas.cz/nuncius/nsiii_03.pdf
| accessdate = 2007-03-13
}}</ref> Pluto has two known smaller moons, ] and ], discovered in 2005.<ref>
{{cite journal
| author = B. Sicardy, W. Beisker et al.
| year = 2006
| title = Observing Two Pluto Stellar Approaches In 2006: Results On Pluto's Atmosphere And Detection Of Hydra
| journal = ]
| volume = 38 |pages =542
| bibcode = 2006DPS....38.3106S
| doi =
}}</ref>

== Discovery ==
{{Main|Planets beyond Neptune}}
]

In the 1840s, using ], ] predicted the position of the then-undiscovered planet ] after analysing perturbations in the orbit of ].<ref>{{cite book
| title = Planet Quest: The Epic Discovery of Alien Solar Systems
| author = K. Croswell
| publisher = The Free Press
| year = 1997
| pages = 43
| isbn = 978-0684832524}}</ref> Subsequent observations of Neptune in the late 19th century caused astronomers to speculate that Uranus' orbit was being disturbed by another planet besides Neptune. In 1906, ], a wealthy Bostonian who had founded the ] in ] in 1894, started an extensive project in search of a possible ninth planet, which he termed "]".<ref name="Tombaugh1946">{{cite journal |last=Tombaugh |first=C. W. |authorlink=Clyde Tombaugh |year=1946 |title=The Search for the Ninth Planet, Pluto |journal=Astronomical Society of the Pacific Leaflets |volume=5 |issue= |pages=73–80 |bibcode=1946ASPL....5...73T }}</ref> By 1909, Lowell and ] had suggested several possible celestial coordinates for such a planet.<ref name="Hoyt"/> Lowell and his observatory conducted his search until his death in 1916, but to no avail. Unbeknownst to Lowell, on March 19, 1915, his observatory had captured two faint images of Pluto, but did not recognise them for what they were.<ref name="Hoyt"/><ref>{{cite book | title=Planets Beyond: Discovering the Outer Solar System | author=Mark Littman | year=1990 | pages=70 | isbn=047151053X | publisher=Wiley }}</ref>

Due to a ten-year legal battle with Constance Lowell, Percival's widow, who attempted to wrest the observatory's million-dollar portion of his legacy for herself, the search for Planet X did not resume until 1929,<ref name=slipher>Croswell, p. 50</ref> when its director, ], summarily handed the job of locating Planet X to ], a 23-year-old Kansas man who had just arrived at the Lowell Observatory after Slipher had been impressed by a sample of his astronomical drawings.<ref name=slipher />

Tombaugh's task was to systematically image the night sky in pairs of photographs taken two weeks apart, then examine each pair and determine whether any objects had shifted position. Using a machine called a ], he rapidly shifted back and forth between views of each of the plates, to create the illusion of movement of any objects that had changed position or appearance between photographs. On February 18, 1930, after nearly a year of searching, Tombaugh discovered a possible moving object on photographic plates taken on January 23 and January 29 of that year. A lesser-quality photograph taken on January 21 helped confirm the movement.<ref>Croswell p. 52</ref> After the observatory obtained further confirmatory photographs, news of the discovery was telegraphed to the ] on March 13, 1930.<ref name="Hoyt">{{cite journal
| title = W. H. Pickering's Planetary Predictions and the Discovery of Pluto
| author = W. G. Hoyt
| journal = Isis
| volume = 67
| issue = 4
| year = 1976
| pages = 551–564.
| url = http://links.jstor.org/sici?sici=0021-1753(197612)67%3A4%3C551%3AWHPPPA%3E2.0.CO%3B2-I
| accessdate = 2007-06-27
| doi = 10.1086/351668
}}</ref>

=== Name ===
]]]
The discovery made headlines across the globe. The ], who had the right to name the new object, received over 1000 suggestions from all over the world, ranging from Atlas to Zymal.<ref name="pluto guide">{{cite web
| url = http://www.space.com/spacewatch/050311_pluto_guide.html
| title = Finding Pluto: Tough Task, Even 75 Years Later
| author = J. Rao
| publisher = SPACE.com
| date=March 11, 2005
| accessdate=2006-09-08
}}</ref> Tombaugh urged Slipher to suggest a name for the new object quickly before someone else did.<ref name="pluto guide"/> Constance Lowell proposed ''],'' then ''Lowell,'' and finally her own first name. These suggestions were disregarded.<ref>{{cite web
| title = The Search Continues
| author = B. Mager
| work = Pluto: The Discovery of Planet X
| url = http://www.discoveryofpluto.com/pluto05.html
| accessdate = 2007-03-27
}}</ref>

The name Pluto was proposed by ] (1918–2009), an eleven-year-old schoolgirl in ], England.<ref name="Venetia">{{cite web
| title = The girl who named a planet
| publisher = BBC News
| author = P. Rincon
| work = Pluto: The Discovery of Planet X
| url = http://news.bbc.co.uk/1/hi/sci/tech/4596246.stm
| date = 2006-01-13
| accessdate = 2007-04-12
}}</ref> Venetia was interested in ] as well as astronomy, and considered the name, that of the ], appropriate for such a presumably dark and cold world. She suggested it in a conversation with her grandfather ], a former librarian at the ]'s ]. Madan passed the name to Professor ], who then cabled it to colleagues in the United States.<ref>{{cite web
| url = http://fredpratt.tripod.com/PR/pluto.html
| title = The Planet 'Pluto'
| author = K. M. Claxton
| publisher = Parents' Union School Diamond Jubilee Magazine, 1891–1951 (Ambleside: PUS, 1951), p. 30–32
| accessdate = 2007-10-15}}</ref>

The object was officially named on March 24, 1930.<ref>{{cite newspaper
| title = The Trans-Neptunian Body: Decision to call it Pluto
| newspaper = ]
| date = May 27, 1930
| pages = 15
}}</ref><ref>{{cite newspaper
| title = Name Pluto Given to Body Believed to Be Planet X
| url = http://select.nytimes.com/gst/abstract.html?res=F60F14FC3D55147A93C7AB178ED85F448385F9&scp=1&sq=Name%20given%20to%20body%20planet%20X&st=cse
| newspaper = ]
| location = New York City
| date = May 25, 1930
| agency = The Associated Press
| issn = 1556067
| page = 1
| quote = Pluto, the title of the Roman gods of the region of darkness, was announced tonight at Lowell Observatory here as the name chosen for the recently discovered trans-Neptunian body, which is believed to be the long-sought Planet X.
}}</ref> Each member of the Lowell Observatory was allowed to vote on a short-list of three: ] (which was already the name for an asteroid), ] (which had lost reputation through being proposed by the unpopular astronomer ]), and Pluto. Pluto received every vote.<ref>Croswell pp. 54–55</ref> The name was announced on May 1, 1930.<ref name="Venetia"/> Upon the announcement, Madan gave Venetia five ] as a reward.<ref name="Venetia"/>

The name was soon embraced by wider culture. The Disney character ], introduced in 1930, was named in the object's honour.<ref>{{cite web|title=Dwarfed by comparison|author=Allison M. Heinrichs|work=Pittsburgh Tribune-Review|url=http://www.pittsburghlive.com/x/pittsburghtrib/news/cityregion/s_467650.html|year=2006|accessdate=2007-03-26}}</ref> In 1941, ] named the newly created ] ] after Pluto, in keeping with the tradition of naming elements after newly discovered planets, such as ], which was named after ], and ], which was named after ].<ref>{{cite web|title=Reflections on the Legacy of a Legend|author=David L. Clark and David E. Hobart|year=2000|url=http://www.fas.org/sgp/othergov/doe/lanl/pubs/00818011.pdf |accessdate=2007-08-09}}</ref>

=== Demise of Planet X ===
], the discoverer of Pluto]]
<div style="float:right; margin:2px;">
{| class=wikitable style="text-align:center; font-size:11px"
|+ Size estimates for Pluto:
!Year!! Mass!!Notes
|-
|1931 || 1 Earth ||] & ]<ref>{{cite journal|title= The Discovery of Pluto |journal= Monthly Notices of the Royal Astronomical Society |volume=91 |month= February | year= 1931 |pages= 380–385 |bibcode= 1931MNRAS..91..380. |accessdate=2008-07-13}}</ref><ref name="Nicholsonetal1930">{{cite journal
| bibcode = 1930PASP...42..350N
| title = The Probable Value of the Mass of Pluto
| last1 = Nicholson
| first1 = Seth B.
| authorlink1 = Seth B. Nicholson
| last2 = Mayall
| first2 = Nicholas U.
| authorlink2 = Nicholas U. Mayall
| journal = Publications of the Astronomical Society of the Pacific
| volume = 42
| issue = 250
| page = 350
| date = December 1930
| doi = 10.1086/124071
}}</ref><ref name="Nicholsonetal1931">{{cite journal
| bibcode = 1931ApJ....73....1N
| title = Positions, Orbit, and Mass of Pluto
| last1 = Nicholson
| first1 = Seth B.
| authorlink1 = Seth B. Nicholson
| last2 = Mayall
| first2 = Nicholas U.
| authorlink2 = Nicholas U. Mayall
| journal = Astrophysical Journal
| volume = 73
| page = 1
| date = January 1931
| doi = 10.1086/143288
}}</ref>
|-
|1948|| .1 (1/10 Earth) || Kuiper <ref>{{cite journal|title= The Diameter of Pluto |author= Gerard P. Kuiper |journal= Publications of the Astronomical Society of the Pacific |volume=62 |issue=366 |pages= 133–137 |month= August | year= 1950 |url= http://articles.adsabs.harvard.edu/full/1950PASP...62..133K
|accessdate= 2008-07-27|doi= 10.1086/126255}}</ref>
|-
|1976||.01 (1/100 Earth)||Cruikshank, Pilcher, & Morrison <ref>Croswell (1997), p. 57.</ref>
|-
|1978||.002 (2/1,000 Earth) || Christy & Harrington <ref name=Christy>{{cite journal |bibcode= 1978AJ.....83.1005C |author= James W. Christy and Robert S. Harrington |title= The Satellite of Pluto |journal= Astronomical Journal |month= August | year= 1978 |volume= 83 |issue= 8 |pages= 1005–1008 |url= http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1978AJ.....83.1005C&amp;data_type=PDF_HIGH&amp;whole_paper=YES&amp;type=PRINTER&amp;filetype=.pdf |doi= 10.1086/112284 }}</ref>
|-
|}</div>
Once found, Pluto's faintness and lack of a resolvable disc cast doubt on the idea that it was Lowell's ]. Estimates of Pluto's mass were revised downward throughout the 20th century. In 1978, the discovery of Pluto's moon ] allowed the measurement of Pluto's mass for the first time. Its mass, roughly 0.2% that of the Earth, was far too small to account for the discrepancies in the orbit of Uranus. Subsequent searches for an alternate Planet X, notably by ],<ref>{{cite web|title=Planet X—The current status|author=P. K. Seidelmann and R. S. Harrington|work=U. S. Naval Observatory|year=1987|url=http://www.springerlink.com/content/r42h4u7232t724uq/ |accessdate=2007-11-04}}</ref> failed. In 1992, Myles Standish used data from '']'''s 1989 flyby of ], which had revised the planet's total mass downward by 0.5%, to recalculate its gravitational effect on Uranus. With the new figures added in, the discrepancies, and with them the need for a Planet X, vanished.<ref name="Standish1993">{{cite journal |title= Planet X—No dynamical evidence in the optical observations |author=Myles Standish |date= 1992-07-16 |url= http://adsbit.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1993AJ....105.2000S |accessdate= 2009-04-30 |journal=Astronomical Journal | volume= 105|issue= 5|pages=200–2006}}</ref> Today, the majority of scientists agree that Planet X, as Lowell defined it, does not exist.<ref>{{cite book|title=The Neptune File|author=Tom Standage|publisher=Penguin|page=168|year=2000}}</ref> Lowell had made a prediction of Planet X's position in 1915 that was fairly close to Pluto's actual position at that time; however, ] concluded almost immediately that this was a coincidence,<ref>{{cite web
| url = http://www.phys-astro.sonoma.edu/people/faculty/tenn/asphistory/1994.html
| title = History I: The Lowell Observatory in 20th century Astronomy
| publisher = The Astronomical Society of the Pacific
| date = 1994-06-28
| accessdate = 2006-03-05}}</ref> a view still held today.<ref name="Standish1993"/>

=== Nomenclature ===
The name ''Pluto'' was chosen in part to evoke the initials of the astronomer ], a desire echoed in the P-L ] that is Pluto's ] (]).<ref>{{Cite web
| title = NASA's Solar System Exploration: Multimedia: Gallery: Pluto's Symbol
| url = http://sse.jpl.nasa.gov/multimedia/display.cfm?IM_ID=263
| publisher = NASA
| accessdate =2007-03-25 }}</ref>
Pluto's ] resembles that of ] (]), but has a circle in place of the middle prong of the trident (]).

In ], ] and ] the name was translated as ''underworld king star'' (冥王星),<!-- please do not add Vietnamese to this list, its name for Pluto uses a different root and is not equivalent to 冥王星, see the cjvlang reference--> <ref name=nineplan>{{cite web |title=Planetary Linguistics |url=http://seds.lpl.arizona.edu/nineplanets/nineplanets/days.html |accessdate=2007-06-12|archiveurl = http://web.archive.org/web/20071217070734/http://seds.lpl.arizona.edu/nineplanets/nineplanets/days.html |archivedate = December 17, 2007|deadurl=yes}}</ref><ref>{{cite web|title=Uranus, Neptune, and Pluto in Chinese, Japanese, and Vietnamese|work=cjvlang.com|url=http://www.cjvlang.com/Dow/UrNepPl.html|accessdate=2008-05-24}}</ref> as suggested by ] in 1930.<ref>{{cite web |title=A Tribute to Houei Nojiri |author=Steve Renshaw and Saori Ihara |url=http://www2.gol.com/users/stever/nojiri.htm |year=2000 |accessdate=2007-06-12}}</ref> Many other non-European languages use a transliteration of "Pluto" as their name for the object; however, some ] use a form of ], the Guardian of Hell in ] mythology, such as the ] ''Yamdev''.<ref name=nineplan />

== Orbit and rotation ==

] to Neptune's orbit (in blue). The ] is horizontal]]
]

Pluto's orbital period lasts for 248 Earth years. Its orbital characteristics are substantially different from those of the planets, which follow nearly circular orbits around the Sun close to a flat reference ] called the ]. In contrast, Pluto's orbit is highly ] relative to the ecliptic (over 17°) and highly ] (]). This high eccentricity means a small region of Pluto's orbit lies nearer the Sun than ]'s. Pluto was last interior to Neptune's orbit between February 7, 1979 and February 11, 1999. Detailed calculations indicate that the previous such occurrence lasted only fourteen years, from July 11, 1735 to September 15, 1749, whereas between April 30, 1483 and July 23, 1503, it had also lasted 20 years.

Although this repeating pattern may suggest a regular structure, in the long term Pluto's orbit is in fact ]. While computer simulations can be used to predict its position for several million years (both ] in time), after intervals longer than the ] of 10–20 million years, calculations become speculative: Pluto's tiny size makes it sensitive to unmeasurably small details of the solar system, hard-to-predict factors that will gradually disrupt its orbit.<ref name="sussman88">{{cite journal
| title = Numerical evidence that the motion of Pluto is chaotic
| author = Gerald Jay Sussman; Jack Wisdom
| journal = Science
| volume = 241
| pages = 433–437
| year = 1988
| bibcode = 1988Sci...241..433S
| doi = 10.1126/science.241.4864.433
| pmid = 17792606
| issue = 4864
}}</ref><ref name="wisdom91">{{cite journal
| title = Symplectic maps for the n-body problem
| author = Jack Wisdom; Matthew Holman
| journal = Astronomical Journal
| volume = 102
| pages = 1528–1538
| year = 1991
| bibcode = 1991AJ....102.1528W
| doi = 10.1086/115978
}}</ref> Millions of years from now, Pluto well may be at ], at ] or anywhere in between, with no way for us to predict which. This does not mean Pluto's orbit itself is unstable, but its position ''on'' that orbit is impossible to determine so far ahead. Several resonances and other dynamical effects keep Pluto's orbit stable, safe from planetary collision or scattering.

=== Relationship with Neptune ===
].]]

Despite Pluto's orbit appearing to cross that of Neptune when viewed from directly above, the two objects' orbits are aligned so that they can never collide or even approach closely. There are several reasons why.

At the simplest level, one can examine the two orbits and see that they do not intersect. When Pluto is closest to the Sun, and hence closest to Neptune's orbit as viewed from above, it is also the farthest above Neptune's path. Pluto's orbit passes about 8 ] above that of Neptune, preventing a collision.<ref name=huainn01>{{cite journal
| title = The 1 : 1 Superresonance in Pluto's Motion
| author = X.-S. Wan, T.-Y. Huang, and K. A. Innanen
| url = http://www.iop.org/EJ/article/1538-3881/121/2/1155/200033.html
| journal = The Astronomical Journal
| volume = 121
| issue = 2
| pages = 1155–1162
| doi = 10.1086/318733
| year = 2001
| accessdate = 2008-09-06
}}</ref><ref name=hunter04>{{cite web
|title = Unmanned scientific exploration throughout the Solar System
|author = Maxwell W. Hunter II
|work = NASA Programs, Lockheed Missiles & Space Company
|url=http://www.springerlink.com/content/gr2261t06700624t/
|year = 2004
|accessdate = 2007-03-28
}}</ref><ref name=malhotra-9planets>{{cite web
|title = Pluto's Orbit
|author = Renu Malhotra
|url = http://www.nineplanets.org/plutodyn.html
|year = 1997
|accessdate = 2007-03-26
}}</ref> Pluto's ], the points at which its orbit crosses the ecliptic, are currently separated from Neptune's by over 21°.<ref>{{cite web
|url = http://nssdc.gsfc.nasa.gov/planetary/factsheet
|author = David R. Williams
|title = Planetary Fact Sheet
|accessdate = 2007-03-31
|publisher = NASA }}</ref>

However, this alone is not enough to protect Pluto; ] from the planets (especially Neptune) could alter aspects of Pluto's orbit (such as its ]) over millions of years so that a collision could be possible. Some other mechanism or mechanisms must therefore be at work. The most significant of these is that Pluto lies in the 3:2 ] with ]: for every three of Neptune's orbits around the Sun, Pluto makes two. The two objects then return to their initial positions and the cycle repeats, each cycle lasting about 500 years. This pattern is configured so that, in each 500-year cycle, the first time Pluto is near ] Neptune is over 50° ''behind'' Pluto. By Pluto's second perihelion, Neptune will have completed a further one and a half of its own orbits, and so will be a similar distance ''ahead'' of Pluto. Pluto and Neptune's minimum separation is over 17&nbsp;AU. Pluto comes closer to ] (11&nbsp;AU) than it does to Neptune.<ref name=malhotra-9planets />

The 3:2 resonance between the two bodies is highly stable, and is preserved over millions of years.<ref name=sp-345>{{cite web |author=Hannes Alfvén and Gustaf Arrhenius |url=http://history.nasa.gov/SP-345/ch8.htm |title=SP-345 Evolution of the Solar System |year=1976 |accessdate=2007-03-28}}</ref> This prevents their orbits from changing relative to one another; the cycle always repeats in the same way, and so the two bodies can never pass near to each other. Thus, even if Pluto's orbit were not highly inclined the two bodies could never collide.<ref name="malhotra-9planets" />

=== Other factors ===
Numerical studies have shown that over periods of millions of years, the general nature of the alignment between Pluto and Neptune's orbits does not change.<ref name=huainn01/><ref name="williams71">{{cite journal| title=Resonances in the Neptune-Pluto System| author= J. G. Williams; G. S. Benson| journal=Astronomical Journal| volume=76| pages=167| year=1971| bibcode=1971AJ.....76..167W | accessdate=2007-04-17 | doi = 10.1086/111100}}</ref> However, there are several other resonances and interactions that govern the details of their relative motion, and enhance Pluto's stability. These arise principally from two additional mechanisms (besides the 3:2 mean motion resonance).

First, Pluto's ], the angle between the point where it crosses the ecliptic and the point where it is closest to the Sun, ] around 90°.<ref name="williams71"/> This means that when Pluto is nearest the Sun, it is at its farthest above the plane of the Solar System, preventing encounters with Neptune. This is a direct consequence of the ],<ref name="huainn01" /> which relates the eccentricity of an orbit to its inclination to a larger perturbing body—in this case Neptune. Relative to Neptune, the amplitude of libration is 38°, and so the angular separation of Pluto's perihelion to the orbit of Neptune is always greater than 52° (= 90°–38°). The closest such angular separation occurs every 10,000 years.<ref name="sp-345" />

Second, the longitudes of ascending nodes of the two bodies—the points where they cross the ecliptic—are in near-resonance with the above libration. When the two longitudes are the same—that is, when one could draw a straight line through both nodes and the Sun—Pluto's perihelion lies exactly at 90°, and it comes closest to the Sun at its peak above Neptune's orbit. In other words, when Pluto most closely intersects the plane of Neptune's orbit, it must be at its farthest beyond it. This is known as the ''1:1 superresonance'', and is controlled by all the Jovian planets.<ref name="huainn01" />

To understand the nature of the libration, imagine a polar point of view, looking down on the ecliptic from a distant vantage point where the planets orbit ]. After passing the ascending node, Pluto is interior to Neptune's orbit and moving faster, approaching Neptune from behind. The strong gravitational pull between the two causes ] to be transferred to Pluto, at Neptune's expense. This moves Pluto into a slightly larger orbit, where it travels slightly slower, according to ]. As its orbit changes, this has the gradual effect of changing the pericentre and longitudes of Pluto (and, to a lesser degree, of Neptune). After many such repetitions, Pluto is sufficiently slowed, and Neptune sufficiently speeded up, that Neptune begins to catch Pluto at the opposite side of its orbit (near the opposing node to where we began). The process is then reversed, and Pluto loses angular momentum to Neptune, until Pluto is sufficiently speeded up that it begins to catch Neptune again at the original node. The whole process takes about 20,000 years to complete.<ref name=malhotra-9planets /><ref name=sp-345 />

=== Rotation ===
Pluto's ], its ], is equal to 6.39 ]s.<ref name=axis>{{cite book|title=Pluto and Charon: The Odd Couple|pages=401–408|year=2007|publisher=SpringerLink|doi=10.1007/978-1-4020-5544-7|work= Introduction to Planetary Science}}</ref> Like ], Pluto rotates on its "side" on its orbital plane, with an axial tilt of 120°, and so its seasonal variation is extreme; at its ]s, one hemisphere is in permanent daylight, while the other is in permanent darkness.<ref name=oregon/>

== Physical characteristics ==
]]]
]
Pluto's distance from Earth makes in-depth investigation difficult. Many details about Pluto will remain unknown until 2015, when the ] spacecraft is expected to arrive there.<ref name="newhorizons">{{cite web
| author =
| title = Space Probe Heads To Pluto—Finally
| publisher = CBS News
| date = 2006-01-19
| url = http://www.cbsnews.com/stories/2006/01/19/tech/main1219891.shtml
| accessdate = 2007-04-14
}}</ref>

=== Appearance and surface ===
Pluto's visual ] averages 15.1, brightening to 13.65 at perihelion.<ref name="Pluto Fact Sheet">{{cite web
| author = D. R. Williams
| title = Pluto Fact Sheet
| publisher = NASA
| date = September 7, 2006
| url =http://nssdc.gsfc.nasa.gov/planetary/factsheet/plutofact.html
| accessdate =2007-03-24
}}</ref> To see it, a telescope is required; around 30&nbsp;cm (12&nbsp;in) aperture being desirable.<ref>{{cite web
| url = http://www.science.edu.sg/ssc/detailed.jsp?artid=1950&type=6&root=6&parent=6&cat=66
| title = This month Pluto's apparent magnitude is m=14.1. Could we see it with an 11" reflector of focal length 3400 mm?
| publisher = Singapore Science Centre
| accessdate = 2007-03-25
| archiveurl = http://web.archive.org/web/20051111151435/http://www.science.edu.sg/ssc/detailed.jsp?artid=1950&type=6&root=6&parent=6&cat=66| archivedate = November 11, 2005}}</ref> It looks star-like and without a visible disk even in large telescopes, because its ] is only 0.11".<!-- citation needed. -->

Distance, and current limits on telescope technology, make it impossible to directly photograph surface details on Pluto.

The earliest maps of Pluto, made in the late 1980s, were brightness maps created from close observations of eclipses by its largest moon, Charon. Observations were made of the change in the total average brightness of the Pluto-Charon system during the eclipses. For example, eclipsing a bright spot on Pluto makes a bigger total brightness change than eclipsing a dark spot. Computer processing of many such observations can be used to create a brightness map. This method can also track changes in brightness over time.<ref>{{cite journal
| title = A Two-Color Map of Pluto Based on Mutual Event Lightcurves
| author = E. F. Young; R. P. Binzel; K. Crane
| journal = Bulletin of the American Astronomical Society
| volume = 32
| pages = 1083
| work = AA(SwRI), AB(M.I.T.), AC (Boulder High School)
| url = http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2000DPS....32.4601Y
| year = 2000
| accessdate = 2007-03-26}}</ref><ref name="Buie1992">{{cite journal
|last=Buie
|first=M. W.
|coauthors=D. J. Tholen and K. Horne
|year=1992
|title=Albedo maps of Pluto and Charon: Initial mutual event results.
|journal=Icarus
|volume=97
|pages=221–227
|url=http://www.boulder.swri.edu/~buie/biblio/pub015.html}}</ref>

Current maps have been produced from images from the ], which offers the highest ] currently available, and show considerably more detail,<ref name="Buie_web_map"/> resolving variations several hundred kilometres across, including polar regions and large bright spots.<ref name="Buie2010_maps">{{cite journal
|last=Buie
|first=Mark W.
|coauthors=W. M. Grundy, E. F. Young, L. A. Young, and S. A. Stern
|year=2010
|title=Pluto and Charon with the Hubble Space Telescope: I. Resolving changes on Pluto's surface and a map for Charon |journal=Astronomical Journal
|volume=139
|pages=1128–1143
|url=http://www.boulder.swri.edu/~buie/biblio/pub073.html
|doi=10.1088/0004-6256/139/3/1128
}}</ref> The maps are produced by complex computer processing, which find the best-fit projected maps for the few pixels of the Hubble images.<ref name="Buie_mapmaking">{{cite web
|url=http://www.boulder.swri.edu/~buie/pluto/mapstory.html
|title=How the Pluto maps were made
|last=Buie
|first=Mark W.
|accessdate=10 February 2010}}</ref> As the two cameras on the ] used for these maps are no longer in service, these will remain the most detailed maps of Pluto until the 2015 flyby of ].<ref name="Buie_mapmaking"/>

These maps, together with Pluto's lightcurve and the periodic variations in its infrared spectra, reveal that Pluto's surface is remarkably varied, with large changes in both brightness and colour.<ref name="Buie2010_lightcurve">{{cite journal
|last=Buie
|first=Mark W.
|coauthors=W. M. Grundy, E. F. Young, L. A. Young, and S. A. Stern
|year=2010
|title=Pluto and Charon with the Hubble Space Telescope: I. Monitoring global change and improved surface propertices from light curves |journal=Astronomical Journal
|volume=139
|pages=1117–1127
|url=http://www.boulder.swri.edu/~buie/biblio/pub072.html
|doi=10.1088/0004-6256/139/3/1117
}}</ref> Pluto is one of the most contrastive bodies in the Solar System, with as much contrast as ] moon ].<ref name="Buie_web_map">{{cite web
|url=http://www.boulder.swri.edu/~buie/pluto/hrcmap.html
|title=Pluto map information
|last=Buie
|first=Mark W.
|accessdate=10 February 2010}}</ref> The colour varies between charcoal black, dark orange and white:<ref name="Hubble2010">{{cite web|url=http://hubblesite.org/newscenter/archive/releases/2010/06/full/|title=New Hubble Maps of Pluto Show Surface Changes|date= February 4, 2010<!-- 01:00 PM (EST)-->|publisher=News Release Number: STScI-2010-06|accessdate=10 February 2010}}</ref> Buie et al. term it "significantly less red than ] and much more similar to the hues seen on ] with a slightly more orange cast".<ref name="Buie2010_maps"/>

Pluto's surface has changed between 1994 and 2002-3: the northern polar region has brightened and the southern hemisphere darkened.<ref name="Hubble2010"/> Pluto's overall redness has also increased substantially between 2000 and 2002.<ref name="Hubble2010"/> These rapid changes are probably related to ], which is expected to be complex due to Pluto's extreme ] and high ].<ref name="Hubble2010"/>

] analysis of Pluto's surface reveals it to be composed of more than 98 percent ] ice, with traces of methane and carbon monoxide.<ref name=tobias>{{cite journal
|title = Surface Ices and the Atmospheric Composition of Pluto
|author = Tobias C. Owen, Ted L. Roush et al.
| journal = Science
|year=1993
| month = August 6
| volume = 261
| issue = 5122
| pages = 745–748
|doi = 10.1126/science.261.5122.745
|url=http://www.sciencemag.org/cgi/content/abstract/261/5122/745
|accessdate=2007-03-29
|pmid = 17757212}}</ref> The face of Pluto oriented toward Charon contains more ] ice, while the opposite face contains more nitrogen and ] ice.<ref>{{cite news
| url=http://www.msnbc.msn.com/id/3077880/
| publisher=MSNBC
| title=Pluto regains its place on the fringe
| author=Alan Boyle
| date=1999-02-11
| accessdate=2007-03-20}}</ref>

=== Structure ===
]
Observations by the ] place Pluto's density at between 1.8 and 2.1&nbsp;g/cm<sup>3</sup>, suggesting its internal composition consists of roughly 50–70 percent rock and 30–50 percent ice by mass.<ref name=Solstation>{{cite web|title=Pluto|work=SolStation|url=http://www.solstation.com/stars/pluto.htm|year=2006|accessdate=2007-03-28}}</ref> Because decay of radioactive minerals would eventually heat the ices enough for the rock to separate from them, scientists expect that Pluto's internal structure is differentiated, with the rocky material having settled into a dense ] surrounded by a ] of ice. The diameter of the core should be around 1,700&nbsp;km, 70% of Pluto's diameter.<ref name=Hussmann2006>{{cite doi|10.1016/j.icarus.2006.06.005}}</ref> It is possible that such heating continues today, creating a subsurface ocean layer of liquid water some {{nowrap|100 to 180 km}} thick at the core–mantle boundary.<ref name=Hussmann2006/><ref>{{cite web|title=The Inside Story|work=New Horizons|url=http://www.pluto.jhuapl.edu/science/everything_pluto/9_insideStory.html|year=2007|accessdate=2007-03-29}}</ref> The ] ''Institute of Planetary Research'' calculated that Pluto's density-to-radius ratio lies in a transition zone, along with Neptune's moon ], between icy satellites like the mid-sized ] and ], and rocky satellites such as Jupiter's ].<ref> </ref>

=== Mass and size ===
]

Pluto's mass is 1.31×10<sup>22</sup>&nbsp;kg, less than 0.24 percent that of the Earth,<ref>{{cite web
| title = Beyond Pluto (extract)
| author = J. Davies
| work = Royal Observatory, Edinburgh
| url = http://assets.cambridge.org/052180/0196/excerpt/0521800196_excerpt.pdf
| year=2001
| accessdate=2007-03-26}}</ref> while its diameter is 2,306 (+/- 20)&nbsp;km, or roughly 66% that of the Moon.<ref name="Buie06"/> Astronomers, assuming Pluto to be Lowell's Planet X, initially calculated its mass based on its presumed effect on Neptune and Uranus. In 1955 Pluto was calculated to be roughly the mass of the Earth, with further calculations in 1971 bringing the mass down to roughly that of Mars.<ref name="Croswellp57">Croswell p. 57</ref> However, in 1976, Dale Cruikshank, Carl Pilcher and David Morrison of the ] calculated Pluto's ] for the first time, finding that it matched that for ] ice; this meant Pluto had to be exceptionally luminous for its size and therefore could not be more than 1 percent the mass of the Earth.<ref name="Croswellp57"/> Pluto's albedo is 1.3–2.0 times greater than that of Earth.<ref name="Pluto Fact Sheet"/>

The discovery of Pluto's satellite ] in 1978 enabled a determination of the mass of the Pluto–Charon system by application of ]. Once Charon's gravitational effect was measured, Pluto's true mass could be determined. Observations of Pluto in occultation with Charon allowed scientists to establish Pluto's diameter, while the invention of ] allowed them to determine its shape accurately.<ref>{{cite journal
| title = Adaptive optics imaging of Pluto–Charon and the discovery of a moon around the Asteroid 45 Eugenia: the potential of adaptive optics in planetary astronomy
| author = L. M. Close, W. J. Merline, D. J. Tholen, T. C. Owen, F. J. Roddier, C. Dumas,
| journal = Proceedings of the International Society for Optical Engineering
| volume = 4007
| pages = 787–795,
| work = European Southern Observatory
| url = http://www.spie.org/scripts/abstract.pl?bibcode=2000SPIE.4007..787C
| year = 2000
| accessdate=2007-03-26}}</ref>

Among the objects of the Solar System, Pluto is smaller and much less massive than the ]s, and at less than 0.2 lunar masses it is also less massive than seven ]: ], ], ], ], Earth's ], ] and ]. Pluto is more than twice the diameter and a dozen times the mass of the ] ], the largest object in the ]. However, it is smaller than the dwarf planet ], a ] discovered in 2005.

=== Atmosphere ===
]—L. Calçada, with ], and Charon and the Sun in the sky.]]
Pluto's ] consists of a thin envelope of ], ], and ] gases, which are derived from the ices of these substances on its surface.<ref>{{cite web
|title= Nitrogen in Pluto's Atmosphere
|author= Ken Croswell|url=http://www.kencroswell.com/NitrogenInPlutosAtmosphere.html
|year=1992
|accessdate=2007-04-27
}}</ref> Its surface pressure ranges from 6.5 to 24 μbar.<ref name=atmosphere2009>
{{cite arXiv
|eprint=0901.4882
|title=Pluto's lower atmosphere structure and methane abundance from high-resolution spectroscopy and stellar occultations
|class=astro-ph.EP
|year=2009
|last1=Lellouch | first1=E.
|last2=Sicardy | first2=B.
|last3=de Bergh | first3=C.
|last4=Käufl | first4=H. -U.
|last5=Kassi | first5=S.
|last6=Campargue | first6=A.
}}</ref> Pluto's elongated orbit is predicted to have a major effect on its atmosphere: as Pluto moves away from the Sun, its atmosphere should gradually freeze out, and fall to the ground. When Pluto is closer to the Sun, the temperature of Pluto's solid surface increases, causing the ices to ] into gas. This creates an ]; much as ] cools the body as it evaporates from the surface of the skin, this sublimation cools the surface of Pluto. Scientists using the ] have recently discovered that Pluto's temperature is about {{convert|43|K|C|0|sp=us|abbr=on}}, 10 K colder than would otherwise be expected.<ref>{{cite web
| url = http://www.cnn.com/2006/TECH/space/01/03/pluto.temp/index.html
| title = Astronomers: Pluto colder than expected
| author = T. Ker
| publisher = Space.com (via CNN.com)
| year = 2006
| accessdate = 2006-03-05
}}</ref>

The presence of methane, a powerful ], in Pluto's atmosphere creates a ], with average temperatures 36&nbsp;K warmer 10&nbsp;km above the surface.<ref name=atmtemp/> The lower atmosphere contains a higher concentration of methane than its upper atmosphere.<ref name=atmtemp>E. Lellouch, B. Sicardy, C. de Bergh (2009). "" (in press). ''Astronomy & Astrophysics''.</ref>

The first evidence of Pluto's atmosphere was found by the ] in 1985, from observations of the ] of a star behind Pluto. When an object with no atmosphere moves in front of a star, the star abruptly disappears; in the case of Pluto, the star dimmed out gradually.<ref>{{cite web
| title = IAUC 4097
| url = http://cfa-www.harvard.edu/iauc/04000/04097.html#Item0
| year = 1985
| accessdate = 2007-03-26}}</ref> From the rate of dimming, the atmospheric pressure was determined to be 0.15&nbsp;], roughly 1/700,000 that of Earth.<ref>{{cite web
| title = The atmospheres of Pluto and other trans-Neptunian objects
| author = R. Johnston
| url = http://www.johnstonsarchive.net/astro/pluto.html
| year = 2006
| accessdate = 2007-03-26
}}</ref> The conclusion was confirmed and significantly strengthened by extensive observations of another similar ] in 1988.

In 2002, another occultation of a star by Pluto was observed and analysed by teams led by Bruno Sicardy of the ],<ref>{{cite journal
| url = http://www.nature.com/nature/journal/v424/n6945/full/nature01766.html
| title = Large changes in Pluto's atmosphere as revealed by recent stellar occultations
| journal = Nature
| volume = 424
| doi = 10.1038/nature01766
| author = B. Sicardy
| coauthors = T. Widemann, et al.
| publisher = Nature
| date = 2003-07-10
| accessdate = 2006-03-05
| pages = 168
| pmid = 12853950
| issue = 6945
}}</ref> ] of ],<ref>{{cite web
| url = http://web.mit.edu/newsoffice/2002/pluto.html
| title = Pluto is undergoing global warming, researchers find
| publisher = Massachusetts Institute of Technology
| date = 2002-10-09
| accessdate = 2007-03-20
}}</ref> and ] of ].<ref>{{cite web
| url = http://www.williams.edu/admin/news/releases.php?id=162
| title = Williams Scientists Contribute to New Finding About Pluto
| publisher = Williams College
| date = 2003-07-09
| accessdate=2007-03-20
}}</ref> Surprisingly, the atmospheric pressure was estimated to be 0.3&nbsp;pascal, even though Pluto was farther from the Sun than in 1988 and thus should have been colder and had a more rarefied atmosphere. One explanation for the discrepancy is that in 1987 the south pole of Pluto came out of shadow for the first time in 120 years, causing extra nitrogen to sublimate from the polar cap. It will take decades for the excess nitrogen to condense out of the atmosphere as it freezes onto the north pole's now permanently dark ice cap.<ref name=britty>{{cite web
| title = Puzzling Seasons and Signs of Wind Found on Pluto
| author = R. R. Britt
| work = Space.com
| url = http://www.space.com/scienceastronomy/pluto_seasons_030709.html
| year = 2003
| accessdate = 2007-03-26
}}</ref> Spikes in the data from the same study revealed what may be the first evidence of wind in Pluto's atmosphere.<ref name=britty/> Another stellar occultation was observed by the MIT-Williams College team of James Elliot, ], and a ] team led by Leslie Young on June 12, 2006 from sites in Australia.<ref>{{cite journal
| author = J. L. Elliot, M. J. Person, A. A. S. Gulbis, E. R. Adams, E. A. Kramer, C. A. Zuluaga, R. E. Pike, J. M. Pasachoff, S. P. Souza, B. A. Babcock, J. W. Gangestad, A. E. Jaskot, P. J. Francis, R. Lucas, A. S. Bosh
| year=2006
| title = The Size of Pluto's Atmosphere As Revealed by the 2006 June 12 Occultation
| bibcode = 2006DPS....38.3102
| journal = American Astronomical Society
| work = E Pasadena Division of Planetary Sciences
|accessdate=2007-04-12}}</ref>

In October 2006, Dale Cruikshank of NASA/Ames Research Center (a New Horizons co-investigator) and his colleagues announced the spectroscopic discovery of ] on Pluto's surface. This ethane is produced from the photolysis or radiolysis (i.e., the chemical conversion driven by sunlight and charged particles) of frozen methane on Pluto's surface and suspended in its atmosphere.<ref name=ethane>
{{cite web
| author = A. Stern
| authorlink = Alan Stern
| date = November 1, 2006
| url = http://pluto.jhuapl.edu/overview/piPerspectives/piPerspective_11_1_2006.php
| title = Making Old Horizons New
| work = The PI's Perspective
| publisher = Johns Hopkins University Applied Physics Laboratory
| accessdate = 2007-02-12
}}</ref>

== Satellites ==
{{Main|Moons of Pluto}}
]
] on ] ]]
]

Pluto has three known ]s: ], first identified in 1978 by astronomer ]; and two smaller moons, ] and ], both discovered in 2005.<ref>Guy Gugliotta. "." ''].'' November 1, 2005. Retrieved on October 10, 2006.</ref>

The Plutonian moons are unusually close to Pluto, compared to other observed systems. Moons could potentially orbit Pluto up to 53% (or 69%, if retrograde) of the ] radius, the stable gravitational zone of Pluto's influence. For example, ] orbits Neptune at 40% of the Hill radius. In the case of Pluto, only the inner 3% of the zone is known to be occupied by satellites. In the discoverers’ terms, the Plutonian system appears to be "highly compact and largely empty",<ref name="Sternetal 2006">{{cite journal | author = S.A. Stern, H.A. Weaver, A.J. Steffl, M.J. Mutchler, W.J. Merline, M.W. Buie, E.F. Young, L.A. Young, J.R. Spencer | title = Characteristics and Origin of the Quadruple System at Pluto | journal = Nature | volume = 439 | pages = 946–948 | year = 2006 | id = {{arxiv|astro-ph|0512599}} required | doi = 10.1038/nature04548 | pmid = 16495992 | issue = 7079}}</ref> although others have pointed out the possibility of additional objects, including a small ring system.<ref name="Steffl 2006"/>
=== Charon ===
{{Main|Charon (moon)}}
The Pluto-Charon system is noteworthy for being the largest of the Solar System's few binary systems, defined as those whose ] lies above the primary's surface (] is a smaller example).<ref>{{cite web | title = Binary Minor Planets | author = Derek C. Richardson and Kevin J. Walsh | work = Department of Astronomy, University of Maryland|url=http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.earth.32.101802.120208?journalCode=earth|year = 2005| accessdate = 2007-03-26}}</ref> This and the large size of Charon relative to Pluto has led some astronomers to call it a dwarf ].<ref>{{cite web | title = Charon's size and an upper limit on its atmosphere from a stellar occultation | author = B. Sicardy et al. | url = http://www.nature.com/nature/journal/v439/n7072/abs/nature04351.html | year = 2006 | accessdate = 2007-03-26}}</ref> The system is also unusual among planetary systems in that each is ] to the other: Charon always presents the same face to Pluto, and Pluto always presents the same face to Charon: from any position on either body, the other is always at the same position in the sky, or always obscured.<ref>{{cite web|title=The Once and Future Pluto|author=Leslie Young|work=Southwest Research Institute, Boulder, Colorado|url=http://www.boulder.swri.edu/~layoung/projects/talks03/IfA-jan03v1.ppt|year=1997|accessdate=2007-03-26}}</ref> Because of this, the rotation period of each is equal to the time it takes the entire system to rotate around its common centre of gravity.<ref name=axis/> Just as Pluto revolves on its side relative to the orbital plane, so the Pluto-Charon system does also.<ref name=oregon>http://abyss.uoregon.edu/~js/ast121/lectures/lec21.html U. Oregon Ast. 121 Lecture notes </ref> In 2007, observations by the ] of patches of ammonia hydrates and water crystals on the surface of Charon suggested the presence of active cryo-geysers.<ref>{{cite web|title=Charon: An ice machine in the ultimate deep freeze|work=Gemini Observatory|year=2007|url=http://www.spaceflightnow.com/news/n0707/17charon/|accessdate=2007-07-18}}</ref>

=== Nix and Hydra ===
{{Main|Nix (moon)|Hydra (moon)}}

Two additional moons of Pluto were imaged by astronomers working with the ] on May 15, 2005, and received ] of S/2005&nbsp;P&nbsp;1 and S/2005&nbsp;P&nbsp;2. The International Astronomical Union officially named Pluto's newest moons ] (or Pluto&nbsp;II, the inner of the two moons, formerly P&nbsp;2) and ] (Pluto&nbsp;III, the outer moon, formerly P&nbsp;1), on June 21, 2006.<ref>{{cite press release | publisher=International Astronomical Union | date=2006-06-21 | title=IAU Circular No. 8723—Satellites of Pluto | url=http://www-int.stsci.edu/~mutchler/documents/IAU_Circular_8723.pdf | accessdate=2007-02-12}}</ref>

These small moons orbit Pluto at approximately two and three times the distance of Charon: Nix at 48,700 kilometres and Hydra at 64,800 kilometres from the barycenter of the system. They have nearly circular ] orbits in the same orbital plane as Charon, and are very close to (but not in) 4:1 and 6:1 mean motion ]s with Charon.<ref>{{cite journal | author= F. R. Ward | coauthors = RM Canup| year = 2006 | month = August 25 | title = Forced Resonant Migration of Pluto's Outer Satellites by Charon | journal = Science | volume = 313 | issue = 5790 | pages = 1107–1109 | doi = 10.1126/science.1127293 | url = http://www.sciencemag.org/cgi/content/abstract/sci;313/5790/1107 | accessdate = 2007-02-12 | pmid = 16825533}}</ref>

Observations of Nix and Hydra to determine individual characteristics are ongoing. Hydra is sometimes brighter than Nix, suggesting either that it is larger or that different parts of its surface may vary in brightness. Sizes are estimated from albedos. The moons' spectral similarity to Charon suggests a 35% albedo similar to Charon's; this value results in diameter estimates of 46 kilometres for Nix and 61 kilometres for the brighter Hydra. Upper limits on their diameters can be estimated by assuming the 4% albedo of the darkest Kuiper Belt objects; these bounds are 137&nbsp;±&nbsp;11&nbsp;km and 167&nbsp;±&nbsp;10&nbsp;km, respectively. At the larger end of this range, the inferred masses are less than 0.3% that of Charon, or 0.03% of Pluto's.<ref name="Weaver 2006">{{cite journal | author = H. A. Weaver |coauthors = S. A. Stern, M. J. Mutchler, A. J. Steffl, M. W. Buie, W. J. Merline, J. R. Spencer, E. F. Young and L. A. Young | year = 2006 | month = February 23 | title = Discovery of two new satellites of Pluto | journal = Nature | volume = 439| issue = 7079 | pages = 943–945| doi = 10.1038/nature04547 | id = {{arxiv|astro-ph|0601018}} | accessdate = 2007-04-03 required | pmid = 16495991}}</ref>

The discovery of the two small moons suggests that Pluto may possess a variable ]. Small body impacts can create debris that can form into planetary rings. Data from a deep optical survey by the ] on the ] suggest that no ring system is present. If such a system exists, it is either tenuous like the ] or is tightly confined to less than 1,000&nbsp;km in width.<ref name="Steffl 2006">{{cite journal | author = Andrew J. Steffl | coauthors = S. Alan Stern | title = First Constraints on Rings in the Pluto System | journal = The Astronomical Journal | volume = 133 | pages = 1485–1489 | doi = 10.1086/511770 | id={{arxiv|astro-ph|0608036}} | year = 2007 required | issue = 4}}</ref>

Similar conclusions have been made from occultation studies.<ref name="Pasachoff 2006">{{cite journal | author = Pasachoff, Jay M. |coauthors = B. A. Babcock, S. P. Souza, J. W. Gangestad, A. E. Jaskot, J. L. Elliot, A. A. S. Gulbis, M. J. Person, E. A. Kramer, E. R. Adams, C. A. Zuluaga, R. E. Pike, P. J. Francis, R. Lucas, A. S. Bosh, D. J. Ramm, J. G. Greenhill, A. B. Giles, and S. W. Dieters | year = 2006 | month = October | title = A Search for Rings, Moons, or Debris in the Pluto System during the 2006 July 12 Occultation | journal = Bull. Am. Astron. Soc. | volume = 38| issue = 3 | pages = 523| bibcode = 2006DPS....38.2502P | accessdate = 2009-03-17 required}}</ref> In imaging the Plutonian system, observations from Hubble placed limits on any additional moons. With 90% confidence, no additional moons larger than 12&nbsp;km (or a maximum of 37&nbsp;km with an albedo of 0.041) exist beyond the glare of Pluto 5 arcseconds from the dwarf planet. This assumes a Charon-like albedo of 0.38; at a 50% confidence level the limit is 8 kilometres.<ref name="Steffl2005">{{cite journal | journal = The Astronomical Journal | author = A.J. Steffl | coauthors = M.J. Mutchler, H.A. Weaver, S.A.Stern, D.D. Durda, D. Terrell, W.J. Merline, L.A. Young, E.F. Young, M.W. Buie, J.R. Spencer | year = 2006 | title = New Constraints on Additional Satellites of the Pluto System | volume = 132 | issue = 2 | pages = 614–619 | url = http://www.iop.org/EJ/article/1538-3881/132/2/614/205104.html | doi = 10.1086/505424}}</ref>

]

{| class="wikitable sortable"
|+ Pluto's satellites, with Earth's ] comparison<ref name="Buie06">{{cite journal |author=Marc W. Buie, William M. Grundy, Eliot F. Young, Leslie A. Young, S. Alan Stern |title=''Orbits and photometry of Pluto's satellites: Charon, S/2005 P1, and S/2005 P2'' |journal=Astronomical Journal |year=2006 |volume=132 |pages=290 |url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2006AJ....132..290B&amp;db_key=AST&amp;data_type=HTML&amp;format=&amp;high=444b66a47d27727 |id={{arxiv|archive=astro-ph|id=0512491}} |doi=10.1086/504422 }}</ref>
|- style="background:#DDEEFF; font-size:11px;"
! abbr="Name" colspan="2" | Name<br />
(])
! abbr="Discovered" | Discovery <br /> Year
! abbr="Diameter" | Diameter <br />(km)
! abbr="Mass" | Mass <br /> (kg)
! abbr="Radius" | Orbital radius (km)<br />(barycentric)
! abbr="Period" | Orbital period (d)
|- style="text-align:center; background:#ccccff"
| '''Pluto''' || {{IPA|/ˈpluːtoʊ/}} || 1930 || 2,390<br />(70%&nbsp;Moon) || 13,050 {{e|18}}<br />(18%&nbsp;Moon) || 2&nbsp;040 <br />(0.6%&nbsp;Moon) || style="background:#eeeeff;" rowspan="2" | 6.3872<br />(25%&nbsp;Moon)
|- style="text-align:center; background:#eeeeff"
| ''']''' || {{IPA|/ˈʃærən/}},<br />{{IPA|/ˈkɛərən/}} || 1978 || 1,205<br />(35%&nbsp;Moon) || 1,520 {{e|18}}<br />(2%&nbsp;Moon) || 17,530 <br />(5%&nbsp;Moon)
|- style="text-align:center; background:#ffffff"
| ''']''' || {{IPA|/ˈnɪks/}} || 2005 || 88 || 1 {{e|18}} <br /> || 48,708 || 24.9
|- style="text-align:center; background:#ffffff"
| ''']''' || {{IPA|/ˈhaɪdrə/}} || 2005 || 72 || .391 {{e|18}} || 64,749 || 38
|}

{{Clear}}

== Origins ==
{{Main|Kuiper belt}}
] ]]

Pluto's origin and identity had long puzzled astronomers. One early hypothesis was that Pluto was an escaped moon of Neptune, knocked out of orbit by its largest current moon, ]. This notion has been heavily criticised because Pluto never comes near Neptune in its orbit.<ref>{{cite web|title=Pluto's Orbit|work=NASA New Horizons|url=http://pluto.jhuapl.edu/science/everything_pluto/16_plutoOrbit.html|year=2007|accessdate=2007-03-26}}</ref>

Pluto's true place in the Solar System began to reveal itself only in 1992, when astronomers found a population of small icy objects beyond Neptune that were similar to Pluto not only in orbit but also in size and composition. This trans-Neptunian population is believed to be the source of many ]s. Astronomers now believe Pluto to be the largest<ref name=wiki-kbo group=note /> member of the ], a somewhat stable ring of objects located between 30 and 50 AU from the Sun. Like other ]s (KBOs), Pluto shares features with ]; for example, the ] is gradually blowing Pluto's surface into space, in the manner of a comet.<ref>{{cite web| title= Colossal Cousin to a Comet?| work=New Horizons| url=http://pluto.jhuapl.edu/science/everything_pluto/8_cousin.html| accessdate=2006-06-23}}</ref> If Pluto were placed as near to the Sun as Earth, it would develop a tail, as comets do.<ref>{{cite web |year= 1999| author= Neil deGrasse Tyson | title=
Space Topics: Pluto Top Ten: Pluto Is Not a Planet | work=The Planetary Society| url=http://www.planetary.org/explore/topics/topten/tyson_pluto_is_not.html| accessdate=2006-06-23}}</ref>

Though Pluto is the largest of the Kuiper belt objects discovered so far, Neptune's moon ], which is slightly larger than Pluto, is similar to it both geologically and atmospherically, and is believed to be a captured Kuiper belt object.<ref>{{cite web|title=Neptune's Moon Triton|work=The Planetary Society|url=http://www.planetary.org/explore/topics/neptune/triton.html|accessdate=2007-03-26}}</ref> Eris (]) is also larger than Pluto but is not strictly considered a member of the Kuiper belt population. Rather, it is considered a member of a linked population called the ].

A large number of Kuiper belt objects, like Pluto, possess a 3:2 orbital resonance with Neptune. KBOs with this orbital resonance are called "]s", after Pluto.<ref>{{cite web|title=The Plutinos|author=David Jewitt|work=University of Hawaii|url=http://www2.ess.ucla.edu/~jewitt/kb/plutino.html|year=2004|accessdate=2007-03-26|archiveurl = http://web.archive.org/web/20070419234021/http://www.ifa.hawaii.edu/~jewitt/kb/plutino.html |archivedate = April 19, 2007}}</ref>

Like other members of the Kuiper belt, Pluto is thought to be a residual ]; a component of the original ] around the ] that failed to fully coalesce into a full-fledged planet. Most astronomers agree that Pluto owes its current position to a ] undergone by Neptune early in the Solar System's formation. As Neptune migrated outward, it approached the objects in the proto-Kuiper belt, setting one in orbit around itself, which became its moon Triton, locking others into resonances and knocking others into chaotic orbits. The objects in the ], a dynamically unstable region beyond the Kuiper belt, are believed to have been placed in their current positions by interactions with Neptune's migrating resonances.<ref>{{cite web
| title=Neptune's Migration into a Stirred–Up Kuiper Belt: A Detailed Comparison of Simulations to Observations
| publisher=Saint Mary’s University | first=Joseph M.
| last=Hahn | year=2005 | accessdate=2008-03-05
| id={{arxiv|astro-ph|0507319v1}}
}}</ref> A 2004 computer model by Alessandro Morbidelli of the ] in ] suggested that the migration of Neptune into the Kuiper belt may have been triggered by the formation of a 1:2 resonance between Jupiter and Saturn, which created a gravitational push that propelled both Uranus and Neptune into higher orbits and caused them to switch places, ultimately doubling Neptune's distance from the Sun. The resultant expulsion of objects from the proto-Kuiper belt could also explain the ] 600&nbsp;million years after the Solar System's formation and the origin of Jupiter's ]s.<ref name=Levison2007>{{cite journal | author=Harold F. Levison, Alessandro Morbidelli, Crista Van Laerhoven et al. | title=Origin of the Structure of the Kuiper Belt during a Dynamical Instability in the Orbits of Uranus and Neptune|year=2007| bibcode=2007arXiv0712.0553L | id={{arxiv|0712.0553}} | doi=10.1016/j.icarus.2007.11.035 | journal=Icarus | volume=196 | pages=258 }}</ref>
It is possible that Pluto had a near-circular orbit about 33 AU from the Sun before Neptune's migration ] it into a resonant capture.<ref name=Malhorta1995>{{cite journal | author=R. Malhotra | title=The Origin of Pluto's Orbit: Implications for the Solar System Beyond Neptune | journal=Astronomical Journal | id={{arXiv|astro-ph|9504036}} | year=1995 | volume=110 | pages=420 | doi=10.1086/117532 | bibcode=1995AJ....110..420M }}</ref> The Nice model requires that there were about a thousand Pluto-sized bodies in the original planetesimal disk; these may have included the bodies which became Triton and Eris.<ref name=Levison2007/>

== Exploration ==
{{Main|New Horizons}}
]'', launched on January 19, 2006]]
]

Pluto presents significant challenges for spacecraft because of its small mass and great distance from Earth. '']'' could have visited Pluto, but controllers opted instead for a close flyby of ] moon Titan, resulting in a trajectory incompatible with a Pluto flyby. '']'' never had a plausible trajectory for reaching Pluto.<ref>{{cite web| url=http://voyager.jpl.nasa.gov/faq.html| title=Voyager Frequently Asked Questions| accessdate=2006-09-08| publisher=Jet Propulsion Laboratory| date=January 14, 2003}}</ref> No serious attempt to explore Pluto by spacecraft occurred until the last decade of the 20th century. In August 1992, ] scientist ] telephoned Pluto's discoverer, ], requesting permission to visit his planet. "I told him he was welcome to it," Tombaugh later remembered, "though he's got to go one long, cold trip."<ref>{{cite web|title=The last world|author=Dava Sobel|work=Discover magazine|url=http://www.findarticles.com/p/articles/mi_m1511/is_n5_v14/ai_13794133|year=1993|accessdate=2007-04-13}} {{dead link| date=June 2010 | bot=DASHBot}}</ref> Despite this early momentum, in 2000, NASA cancelled the '']'' mission, citing increasing costs and launch vehicle delays.<ref>{{cite web|title=Pluto Kuiper Express|author=Dr. David R. Williams |work=NASA Goddard Space Flight Center|url=http://nssdc.gsfc.nasa.gov/database/MasterCatalog?sc=PLUTOKE|year=2005|accessdate=2007-03-26}}</ref>

After an intense political battle, a revised mission to Pluto, dubbed '']'', was granted funding from the US government in 2003.<ref>{{cite web|title=Pluto Mission a Go! Initial Funding Secured|author=Robert Roy Britt|work=space.com|url=http://www.space.com/scienceastronomy/pluto_horizons_030225.html|year=2003|accessdate=2007-04-13}}</ref> ''New Horizons'' was launched successfully on January 19, 2006. The mission leader, ], confirmed that some of the ashes of Clyde Tombaugh, who died in 1997, had been placed aboard the spacecraft.<ref>{{cite web|title=Happy 100th Birthday, Clyde Tombaugh|author=Dr. Alan Stern|work=Southwest Research Institute|url=http://www.jhuapl.edu/newscenter/pressreleases/2006/060203.asp|year=2006|accessdate=2007-04-13}}</ref>

In early 2007 the craft made use of a ] from ]. Its closest approach to Pluto will be on July 14, 2015; scientific observations of Pluto will begin 5 months before closest approach and will continue for at least a month after the encounter. ''New Horizons'' captured its first (distant) images of Pluto in late September 2006, during a test of the Long Range Reconnaissance Imager (LORRI).<ref>{{cite web | url=http://pluto.jhuapl.edu/news_center/news/112806.php | title=New Horizons, Not Quite to Jupiter, Makes First Pluto Sighting | publisher=The Johns Hopkins University Applied Physics Laboratory | date=2006-11-28 | accessdate=2009-12-25}}</ref> The images, taken from a distance of approximately 4.2 billion kilometres, confirm the spacecraft's ability to track distant targets, critical for maneuvering toward Pluto and other Kuiper Belt objects.

''New Horizons'' will use a remote sensing package that includes imaging instruments and a radio science investigation tool, as well as spectroscopic and other experiments, to characterise the global geology and morphology of Pluto and its moon Charon, map their surface composition and analyse Pluto's neutral atmosphere and its escape rate. ''New Horizons'' will also photograph the surfaces of Pluto and Charon.

Discovery of moons Nix and Hydra may present unforeseen challenges for the probe. Debris from collisions between Kuiper belt objects and the smaller moons, with their relatively low escape velocities, may produce a tenuous dusty ring. Were New Horizons to fly through such a ring system, there would be an increased potential for {{dp|micrometeoroid}} damage that could disable the probe.<ref name="Steffl 2006"/>
{{Clear}}

== Classification ==
{{main|Definition of planet}}
{{TNO imagemap}}
After Pluto's place within the Kuiper belt was determined, its official status as a planet became controversial, with many questioning whether Pluto should be considered together with or separately from its surrounding population.

Museum and planetarium directors occasionally created controversy by omitting Pluto from planetary models of the Solar System. The ] reopened after renovation in 2000 with a model of only eight planets. The controversy made headlines at the time.<ref>{{cite web| url=http://www.space.com/scienceastronomy/solarsystem/tyson_responds_010202.html| title=Astronomer Responds to Pluto-Not-a-Planet Claim| date=February 2, 2001| publisher=Space.com| Niel deGrasse Tyson| accessdate=2006-09-08}}</ref>

In 2002, the KBO ] was discovered, with a diameter then thought to be roughly 1280 kilometres, about half that of Pluto.<ref>{{cite web|title=Direct Measurement of the Size of the Large Kuiper Belt Object (50000) Quaoar|author=Michael E. Brown and Chadwick A. Trujillo|work=The American Astronomical Society|url=http://www.journals.uchicago.edu/cgi-bin/resolve?doi=10.1086/382513|year=2006|accessdate=2007-03-26}} {{dead link| date=June 2010 | bot=DASHBot}}</ref> In 2004, the discoverers of ] placed an upper limit of 1800&nbsp;km on its diameter, nearer to Pluto's diameter of 2320&nbsp;km,<ref>{{cite web|title=Diverse Albedos of Small Trans-Neptunian Objects|author=W. M. Grundy, K. S. Noll, D. C. Stephens|work=Lowell Observatory, Space Telescope Science Institute|id={{arxiv|astro-ph|0502229}}|accessdate=2007-03-26}}</ref> although Sedna's diameter was revised downward to less than 1600&nbsp;km by 2007.<ref name=spitzer>{{cite web |title=Physical Properties of Kuiper Belt and Centaur Objects: Constraints from Spitzer Space Telescope |first=John |last=Stansberry |coauthors=Will Grundy, Mike Brown, Dale Cruikshank, John Spencer, David Trilling, Jean-Luc Margot |work=University of Arizona, Lowell Observatory, California Institute of Technology, NASA Ames Research Center, Southwest Research Institute, Cornell University |id={{arxiv|astro-ph|0702538v2}} |year=2007 |accessdate=2009-03-17}}</ref> Just as ] eventually lost its planet status after the discovery of the other ]s, so, it was argued, Pluto should be reclassified as one of the Kuiper belt objects.

On July 29, 2005, the discovery of a new ] was announced. Named ], it is now known to be slightly larger than Pluto.<ref>{{cite web|title=Hubble Finds 'Tenth Planet' Slightly Larger Than Pluto|work=Hubblesite|url=http://hubblesite.org/newscenter/newsdesk/archive/releases/2006/16/|year=2006|accessdate=2007-03-26}}</ref> This was the largest object discovered in the Solar System since ] in 1846. Its discoverers and the press initially called it the ], although there was no official consensus at the time on whether to call it a planet.<ref>{{cite web|title=NASA-Funded Scientists Discover Tenth Planet|work=Jet Propulsion Laboratory|url=http://www.jpl.nasa.gov/news/news-print.cfm?release=2005-126| year=2005|accessdate=2007-02-22}}</ref> Others in the astronomical community considered the discovery the strongest argument for reclassifying Pluto as a minor planet.<ref>{{cite web| id={{arxiv|astro-ph|0608359}}| title= What is a Planet?|author=Steven Soter| date=2006-08-16| accessdate=2006-08-24}} submitted to The Astronomical Journal, August 16, 2006</ref>

=== 2006: IAU classification ===
{{Main|IAU definition of planet}}

The debate came to a head in 2006 with an ] that created an official definition for the term "planet". According to this resolution, there are three main conditions for an object to be considered a 'planet':
# The object must be in orbit around the ].
# The object must be massive enough to be a sphere by its own gravitational force. More specifically, its own gravity should pull it into a shape of ].
# It must have ] around its orbit.<ref>{{cite news|url=http://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf|title=IAU 2006 General Assembly: Resolutions 5 and 6|date=August 24, 2006|publisher=IAU}}</ref><ref name=IAU0603>{{cite press release |date=2006-08-24 |publisher=International Astronomical Union (News Release—IAU0603) |title=IAU 2006 General Assembly: Result of the IAU Resolution votes |url=http://www.iau.org/public_press/news/release/iau0603/ |accessdate=2008-06-15}}</ref>

Pluto fails to meet the third condition, since its mass was only 0.07 times that of the mass of the other objects in its orbit (Earth's mass, by contrast, is 1.7 million times the remaining mass in its own orbit).<ref>{{cite web|title=What is a Planet?|author= Steven Soter|work=Department of Astrophysics, American Museum of Natural History|url=http://www.sciam.com/article.cfm?chanID=sa006&articleID=93385350-E7F2-99DF-3FD6272BB4959038&pageNumber=2&catID=2|year=2007|accessdate=2007-02-21}}</ref><ref>{{cite news|url=http://www.iau2006.org/mirror/www.iau.org/iau0603/index.html|title=IAU 2006 General Assembly: Result of the IAU Resolution votes|date=August 24, 2006|publisher=IAU}}</ref> The IAU further resolved that Pluto be classified in the simultaneously created ] category, and that it act as the prototype for the ] category of ]s, in which it would be separately, but concurrently, classified.<ref name="IAU0804">{{cite web |date=June 11, 2008, Paris |title=Plutoid chosen as name for Solar System objects like Pluto |publisher=] (News Release—IAU0804) |url=http://www.iau.org/public_press/news/release/iau0804 |accessdate=2008-06-11}}</ref>

On September 13, 2006, the IAU included Pluto, ], and the Eridian moon ] in their ], giving them the official minor planet designations "(134340) Pluto", "(136199) Eris", and "(136199) Eris I Dysnomia".<ref>{{cite web|title=Circular No. 8747|author=Central Bureau for Astronomical Telegrams, International Astronomical Union|url=http://www.cfa.harvard.edu/iau/special/08747.pdf|year=2006|accessdate=2007-02-23|archiveurl = http://web.archive.org/web/20070205035336/http://www.cfa.harvard.edu/iau/special/08747.pdf |archivedate = February 5, 2007}}</ref> If Pluto had been given a ] upon its discovery, the number would have been a little over a thousand rather than over 100,000.

There has been some resistance within the astronomical community toward the reclassification.<ref name="geoff2006c">{{cite news| url=http://space.com/scienceastronomy/060824_planet_definition.html| title=Pluto Demoted: No Longer a Planet in Highly Controversial Definition| author=Robert Roy Britt| publisher=Space.com| date=August 24, 2006| accessdate=2006-09-08}}</ref><ref name="Ruibal-1999">{{cite news |author = Sal Ruibal | title = Astronomers question if Pluto is real planet | publisher = ] | date = January 6, 1999}}</ref><ref name="Britt-2006">{{cite news | author = Robert Roy Britt | url = http://www.space.com/scienceastronomy/061121_exoplanet_definition.html | title = Why Planets Will Never Be Defined | publisher = Space.com | date = November 21, 2006 | accessdate = 2006-12-01}}</ref> ], principal investigator with ]'s '']'' mission to Pluto, has publicly derided the IAU resolution, stating that "the definition stinks, for technical reasons."<ref name="geoff2006a">{{cite news| url=http://msnbc.msn.com/id/14489259/| title=Scientists decide Pluto’s no longer a planet| author=Robert Roy Britt| date= August 24, 2006| publisher=MSNBC| accessdate=2006-09-08}}</ref> Stern's contention is that by the terms of the new definition Earth, Mars, Jupiter, and Neptune, all of which share their orbits with asteroids, would be excluded.<ref name="newscientistspace">{{cite news| url=http://www.newscientistspace.com/article/dn9846-new-planet-definition-sparks-furore.html| title=New planet definition sparks furore| date=August 25, 2006| publisher=NewScientist.com| author= David Shiga| accessdate=2006-09-08}}</ref> <!--Reference does not verify this sentence: "However, his own published writing has ] the new list of planets, as "our Solar System clearly contains" eight planets that have cleared their neighbourhoods."<ref name="Stern 2002">{{cite journal | author=S. Alan Stern | coauthors=and Harold F. Levinson| year=2002 | title=Regarding the criteria for planethood and proposed planetary classification schemes | url=http://www.boulder.swri.edu/~hal/PDF/planet_def.pdf ] | journal=Highlights of Astronomy| volume=12 | pages=205–213, as presented at the XXIVth General Assembly of the IAU–2000 }}</ref>--> His other claim is that because less than five percent of astronomers voted for it, the decision was not representative of the entire astronomical community.<ref name="newscientistspace" /> ] of the Lowell observatory has voiced his opinion on the new definition on his website and is one of the petitioners against the definition.<ref>{{cite web | url=http://www.boulder.swri.edu/~buie/pluto/iauresponse.html | title=My response to 2006 IAU Resolutions 5a and 6a | publisher=Lowell Observatory | author=Marc W. Buie | month=September | year=2006 | accessdate=2007-03-20|archiveurl = http://web.archive.org/web/20070603104622/http://www.lowell.edu/users/buie/pluto/iauresponse.html |archivedate = June 3, 2007}}</ref> Others have supported the IAU. Mike Brown, the astronomer who discovered ], said "through this whole crazy circus-like procedure, somehow the right answer was stumbled on. It’s been a long time coming. Science is self-correcting eventually, even when strong emotions are involved."<ref name="geoff2006b">{{cite news | url=http://www.nytimes.com/2006/08/24/science/space/25pluto.html?ei=5087&en=cfe4d03207c823f2&ex=1172030400&adxnnl=1&excamp=GGGNpluto&adxnnlx=1156820936-x7vi0zUxIJHoKC1TQ0qrMA | title=Pluto Is Demoted to ‘Dwarf Planet’ | publisher=The New York Times | author=Dennis Overbye | date=2006-08-24 | accessdate=2007-03-20}}</ref>

Researchers on both sides of the debate gathered on August 14–16, 2008, at ] for a conference that included back-to-back talks on the current IAU definition of a planet.<ref name=Hopkins>{{cite web |date=April 10, 2008 |title=Is Rekindling the Pluto Planet Debate a Good Idea? |publisher=Scientific American |author=JR Minkel |url=http://www.sciam.com/article.cfm?id=rekindling-the-pluto-planet-debate |accessdate=2008-04-13}}</ref> Entitled "The Great Planet Debate",<ref></ref> the conference published a post-conference press release indicating that scientists could not come to a consensus about the definition of a planet.<ref>Planetary Science Institute press release on September 19th, 2008 "Scientists Debate Planet Definition and Agree to Disagree" </ref> Just before the conference, on June 11, 2008, the IAU announced in a press release that the term "]" would henceforth be used to describe Pluto and other objects similar to Pluto which have an orbital ] greater than that of Neptune and enough mass to be of near-spherical shape.<ref>News Release—IAU0804: Plutoid chosen as name for Solar System objects like Pluto, </ref><ref>Discover Magazine, January 2009 p.76 "Plutoids Join the Solar Family"</ref><ref>Science News, July 5th, 2008 p.7</ref>

=== Public reaction to the change ===
]

Reception to the IAU decision was mixed. While some accepted the reclassification, others seek to overturn the decision with online petitions urging the IAU to consider reinstatement. A resolution introduced by some members of the California state assembly light-heartedly denounces the IAU for "scientific heresy," among other crimes.<ref>{{cite news| url=http://www.space.com/searchforlife/060907_pluto_politics.html|title= Planetary Politics: Protecting Pluto| first= Edna| last= DeVore| date=September 7, 2006| publisher=Space.com| accessdate=2006-09-08}}</ref> The U.S. state of ]'s ] passed a resolution in honor of Tombaugh, a longtime resident of that state, which declared that Pluto will always be considered a planet while in New Mexican skies and that March 13, 2007 was Pluto Planet Day.<ref>{{cite journal | title = Rehabilitating Pluto | author = C. Holden | journal = Science | volume = 315 | pages = 1643 | url = http://www.sciencemag.org/cgi/reprint/315/5819/1643b.pdf | year = 2007 | doi = 10.1126/science.315.5819.1643c | accessdate=2007-04-13}}</ref><ref>{{cite web|url=http://legis.state.nm.us/Sessions/07%20Regular/memorials/house/HJM054.html|title=A joint memorial. Declaring Pluto a planet and declaring March 13, 2007, "Pluto planet day" at the legislature|author=Gutierrez, Joni Marie|publisher=Legislature of New Mexico|accessdate=2009-09-05|year=2007}}</ref> The ] ] passed a similar resolution in 2009, on the basis that Clyde Tombaugh, the discoverer of Pluto, was born in Illinois. The resolution asserted that Pluto was "unfairly downgraded to a 'dwarf' planet" by the IAU.<ref></ref>

Some members of the public have also rejected the change, citing the disagreement within the scientific community on the issue, or for sentimental reasons, maintaining that they have always known Pluto as a planet and will continue to do so regardless of the IAU decision.<ref name="iol">"{{cite news| url=http://www.iol.co.za/index.php?set_id=1&click_id=31&art_id=qw1161415265563B221| title=Pluto's still the same Pluto| date=October 21, 2006| publisher=IOL.co.za| accessdate=2006-11-01}}</ref>

=== Plutoed ===
The verb "to pluto" (] and ]: "'''plutoed'''") was a ] coined in the aftermath of its transition from planet to dwarf planet in the aftermath of the 2006 IAU decision. In January 2007, the ] chose "plutoed" as its 2006 Word of the Year, defining "''to pluto''" as "''to demote or devalue someone or something''", "as happened to the former planet Pluto when the General Assembly of the International Astronomical Union decided Pluto no longer met its definition of a planet."<ref>{{cite web| url=http://www.americandialect.org/Word-of-the-Year_2006.pdf|title= “Plutoed” Voted 2006 Word of the Year| date=January 5, 2007| publisher=American Dialect Society| accessdate=2007-01-07}}</ref><ref name="cnn">{{Cite web|url=http://www.cnn.com/2007/US/01/07/word.of.the.year/index.html|title=Pluto's revenge: 'Word of the Year' award|accessdate=2007-01-20|publisher=CNN|date=January 7, 2007}}</ref>

Society president Cleveland Evans stated the reason for the organization's selection of "plutoed": "Our members believe the great emotional reaction of the public to the demotion of Pluto shows the importance of Pluto as a name. We may no longer believe in the ], but we still have a sense of connection with the former planet."<ref name="msnbc">{{Cite web|url=http://www.msnbc.msn.com/id/16529756|title='Plutoed' chosen as '06 Word of the Year|accessdate=2007-01-10|publisher=Associated Press|year=Jan. 8, 2007}}</ref>

== See also ==
{{Portal|Solar System}}
{{Misplaced Pages-Books|Solar System}}
* ]
* ]
* ]
* ]
* ]
* ]
* ] (section of ])

== Notes ==
{{Reflist|colwidth=60em|group=note}}

== References ==
{{Reflist|colwidth=30em}}

== External links ==
{{Commons category|Pluto}}
{{Sisterlinks|Pluto}}
* {{cite web |last=Nunberg |first=G. |title=Dwarfing Pluto |publisher=NPR |url=http://www.npr.org/templates/story/story.php?storyId=5723794 |date=August 28, 2006 |accessdate=2007-04-13 }} An examination of the redefinition of Pluto from a linguistic perspective.
* at
* {{cite web |title=Lowell Observatory |url=http://www.lowell.edu/AboutLowell/history.html |year=2007 |accessdate=2007-04-13 | archiveurl = http://web.archive.org/web/20070311043927/http://www.lowell.edu/AboutLowell/history.html| archivedate = March 11, 2007}} Website of the observatory that discovered Pluto
* {{cite web |last=Williams |first=D. R. |title=Pluto Fact Sheet |publisher=NASA |url=http://nssdc.gsfc.nasa.gov/planetary/factsheet/plutofact.html |date=September 7, 2006 |accessdate=2007-04-13 }}
* {{cite web |last=Dunn |first=T. |title=Pluto's 3:2 Resonance with Neptune |publisher=Gravity Simulator |url=http://www.orbitsimulator.com/gravity/articles/pluto.html |year=2006 |accessdate=2007-04-13 }}
* {{cite web |last=Fraknoi |first=A. |title=Teaching What a Planet Is: A Roundtable on the Educational Implications of the New Definition of a Planet |publisher=Astronomy Education Review |url=http://aer.noao.edu/cgi-bin/article.pl?id=207 |year=2006 |accessdate=2007-04-13 }} Series of personal articles written by astronomers involved in the debate.
* {{cite web |title=Pluto: Planet or Impostor? |url=http://www.ianridpath.com/Pluto.pdf |month=December | year=1978 |accessdate=2008-11-21 }} An early article questioning the planetary status of Pluto.
* —entries in the ]
*
* (] synthetic computer simulation of how Pluto would appear now)
* from ] on ]
*
*
*
* ] Pluto crossed in front of the dark ] nebula in Sagittarius on 7 July 2010, making it temporarily easier to observe in the rich star fields of the Milky Way

{{Pluto}}
{{Trans-Neptunian dwarf planets}}
{{Trans-Neptunian objects}}
{{MinorPlanets Navigator|(134339) 5628 T-3|(134341) 1979 MA|PageName=(134340) Pluto|state=autocollapse}}
{{Solar System}}

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Revision as of 07:59, 22 September 2010

Not a planet.

Yep, definately not a planet